This invention relates generally to recovering viscous petroleum from petroleum-containing formations. Throughout the world there are several major deposits of high-viscosity crude petroleum in oil sands not recoverable in their natural state through a well by ordinary production methods. In the United States, the major concentration of such deposits is in Utah, where approximately 26 billion barrels of in-place heavy oil or tar exists. In California, the estimate of in-place heavy oil or viscous crude is 220 million barrels. By far the largest deposits in the world are in the Province of Alberta, Canada, and represent a total in-place resource of almost 1000 billion barrels. The depths range from surface outcroppings to about 2000 feet.
To date, none of these deposits has been produced commercially by an in-situ technology. Only one commercial mining operation exists, and that is in a shallow Athabasca deposit. A second mining project is about 75% completed at the present time. However, there have been many in-situ well-to-well pilots, all of which used some form of thermal recovery after establishing communication between injector and producer. Normally such communication has been established by introducing a pancake fracture. The displacing or drive mechanism has been steam and combustion, such as the project as Gregoire Lake or steam and chemicals such as the early work on Lease 13 of the Athabasca deposit. Another means of developing communication is that proposed for the Peace River project. It is expected to develop well-to-well communication by injecting steam over a period of several years into an acquifer underlying the tar sand deposit at a depth of around 1800 feet. Probably the most active in-situ pilot in the oil sands has been that at Cold Lake. This project uses the huff-and-puff single-well method of steam stimulation and has been producing about 4000 barrels of viscous petroleum per day for several years from about 50 wells. This is probably a semi-commercial process, but whether it is a paying proposition is unknown.
The most difficult problem in any in-situ well-to-well viscous petroleum project is establishing and maintaining communication between injector and producer. In shallow deposits, fracturing to the surface has occurred in a number of pilots so that satisfactory drive pressure could not be maintained. In many cases, problems arise from healing of the fracture when the viscous petroleum that had been mobilized through heat cooled as it moved toward the producer. The cool petroleum is essentially immobile, since its viscosity in the Athabasca deposits, for example, is on the order of 100,000 to 1,000,000 cp at reservoir temperature.
As noted, the major problem of the economic recovery from many formations has been establishing and maintaining communication between an injection position and a recovery position in the viscous oil-containing formation. This is primarily due to the character of the formations, where effective mobility of fluids may be extremely low, and in some cases, such as the Athabasca Tar Sands, virtually nil. Thus, the Athabasca Tar Sands, for example, are strip mined where the overburden is limited. In some tar sands, hydraulic fracturing has been used to establish communication between injectors and producers. This has not met with uniform success. A particularly difficult situation develops in the intermediate overburden depths, which cannot stand fracturing pressure.